Neuropeptide Y (NPY) is a 36 amino acid peptide which is contained in and co-released with norepinephrine (NE) from sympathetic nerves innervating vascular and cardiac tissues. Experiments performed with conscious Sprague Dawley rats demonstrated that an intravenous infusion of NPY decreased cardiac output (CO), stroke volume (SV), heart rate (HR) and increased mean arterial pressure (MAP) and total peripheral resistance (TPR). An infusion of NE increased SV, MAP, TPR, with a decrease in HR and no change in CO. Intraventricular pressure measurements in anesthetized rats showed that with an equivalent increase in MAP, the concomitant increase in left ventricular end diastolic pressure was twice as great with NPY as compared to NE. NPY decreased dP/dt while NE induced a significant increase. In contrast to NE, NPY possesses both negative inotropic and chronotropic activity and may modulate the cardiac response to NE.